Cquery (Configurable Query)

Overview

cquery is a command complementary to query that properly handles
configurations. While cquery returns answers that are more correct, it does not
replace query as it does not support all of query's functions.

cquery achieves configuration awareness by running after the
analysis phase
of a build, unlike traditional query, which runs after the loading phase.

The most common use for cquery is obtaining answers that correctly evaluate
select()
statements in configurable attributes. For example:

Keep in mind that cquery works only on the configured target graph. Because of this,
it does not have insight into artifacts like build actions nor access to
test_suite
rules as they are not configured targets.

Basic Syntax

A simple example of the syntax for cquery is as follows:

bazel cquery "function(//target)"

The query expression (in quotes) consists of the following:

function(...) is the function to run on the target. cquery
supports most of the same functions
as traditional query.

//target is the expression fed to the function. In this example, the
expression is a simple target, but the query language also allows nesting of functions.
See the Query How-To for examples.

Note that cquery requires a target on which to run the
loading and analysis phases. Unless otherwise specified, cquery parses
the target(s) listed in the query expression. See the --universe_scope
option below for querying targets built under other targets.

Functions

Of the set of functions
supported by the traiditional query, cquery supports all but siblings, buildfiles, and tests. The
functions listed below are cquery-specific.

config

expr ::= config(expr, word)

The config operator attempts to return the result of the first argument, configured
in the configuration specified by the second argument. Today, the second argument can only take in
three options target, host, or null but we hope to expand
this functionality to be able to input custom configurations (or custom configuration diffs from
the default target configuration).

$ bazel cquery config(//foo, host) --universe_scope=//bar

Note that the above example query will return //foo configured in the host configuration
if and only if//foo exists in the host configuration in the universe of
this query (which we've set to the transitive closure of //bar).

If not all results of the first argument can be found in the specified
configuration, then only those that can be found are returned. If no results of the
first argument can be found in the specified configuration, an error is thrown.

Options

Build Options

cquery runs on top of a regular Bazel build and thus inherits the set of
options
available during a build.

Cquery options

--universe_scope (comma-separated list)

Often, the dependencies of configured targets go through
transitions,
which causes their configuration to differ from their dependent. This flag allows you to query
a target as if it were built as a dependency or a transitive dependency of another target. For
example:

Genrules configure their tools in the
host configuration
so the following queries would produce the following outputs:

Query

Target Built

Output

bazel cquery "//x:tool"

//x:tool

//x:tool(targetconfig)

bazel cquery "//x:tool" --universe_scope="//x:my_gen"

//x:my_gen

//x:tool(hostconfig)

If this flag is set, its contents are built. If it's not set, all targets
mentioned in the query expression are built instead. The transitive closure of the
built targets are used as the universe of the query. Either way, the targets to
be built must be buildable at the top level (that is, compatible with top-level
options). cquery returns results in the transitive closure of these
top-level targets.

Even if it's possible to build all targets in a query expression at the top
level, it may be beneficial to not do so. For example, explicitly setting
--universe_scope could prevent building targets multiple times in
configurations you don't care about. It could also help specify which configuration version of a
target you're looking for (since it's not currently possible
to fully specify this any other way). We recommend that you set this
flag if your query expression is more complex than deps(//foo).

--implicit_deps (boolean, default=True)

Setting this flag to false filters out all results that aren't explicitly set in
the BUILD file and instead set elsewhere by Bazel.

--host_deps (boolean, default=True)

Setting this flag to false filters out all configured targets for which the
path from the queried target to them crosses a transition between the target
configuration and the
host configuration.
If the queried target is in a non-host configuration, setting --nohost_deps will
only return targets that also are in non-host configurations. If the queried
target is in a host configuration, setting --nohost_deps will only return
targets also in the host configuration.

Output Formats

By default, cquery outputs results in a dependency-ordered list of label and configuration pairs.
There are other options for exposing the results as well.

Transitions

Configuration transitions
are used to build targets underneath the top level targets in different configurations than the top
level targets. For example, a target might impose a transition to the host transition on all
dependencies in its tools attribute. These are known as attribute transitions.
Rules can also impose transitions on their own configurations, known as rule class transitions.
This output format outputs information about these transitions such as what type they are and the
effect they have on build options.

This output format is triggered by the --transitions flag which by default is set to
NONE. It can be set to FULL or LITE mode. FULL
mode outputs information about rule class transitions and attribute transitions including a detailed
diff of the options before and after the transition. LITE mode outputs the same information
without the options diff.

Known Issues

Inaccessible configurations. With the exception of the host configuration being
printed as HOST, Configurations are currently output as
hashes and there is no way for the user to input them (and thus to directly
specify the configuration to query a target in).

Non-deterministic output.Cquery does not automatically wipe the build
graph from previous commands and is therefore prone to picking up results
from past queries. For example, genquery exerts a host transition on its tools
attribute - that is, it configures its tools in the
host configuration.
We can see the lingering effects of that transition below.